Congestion of the electromagnetic frequency spectrum is becoming increasingly severe as the communication needs of society expands. This overcrowding is particularly acute in the frequency bands allocated to land-mobile radio services and to the so-called citizen's bands in large metropolitan areas. There are many areas in which the spectrum is being used to its capacity, so far as conventional techniques are concerned, and no further expansion of service is possible. Since these congested bands include those allocated to public safety (police, fire, ambulance, etc.), the existing limitations have serious implications with regard to the ability of such communication systems to truly meet the needs under conditions of natural disaster or national emergency.
In addition to the question of meeting immediate needs, there is little or no capability for expanding existing services to cope with population increases, or to add new services that modern technology makes possible. An example of the latter is the widespread use of personal radio telephones; a service that is certainly possible now from a technology standpoint but is not possible from the standpoint of existing spectrum allocations. It is easy to conceive of many other new services that a highly-mobile and technically-oriented society may demand.
It is clear that if the communication needs of the future are to be met in a satisfactory manner, some dramatic changes in communication technology are called for. Furthermore, these changes must take advantage of technology's emerging capability for implementing complex systems that are small, light, efficient and inexpensive. Finally, the artificial constraints imposed by obsolete legal requirements must be removed in portions (if not all) of the electromagnetic spectrum.
The most direct method of alleviating spectrum congestion is to allocate a larger portion of the spectrum to those services that need it most. While this approach is possible to some extent (and, in fact, has already been done in the 900 MHz region) it does not face up to the basic question of utilizing the spectrum more efficiently.
A second method (which will probably be implemented at some time in the future) is to require all communications between non-mobile units to utilize either a non-radiative transmission medium such as coaxial cable or a directive microwave link. This would reserve most of the electromagnetic spectrum for those services in which one or both users of a given channel are mobile.
For mobile service, a third approach that has been widely discussed in the literature is the use of cellular systems in which communication within each cell is restricted to transmission between any mobile unit and a base station. All base stations may be connected via land lines or microwave links to a central processing unit where all system control functions are coordinated and executed. It may be readily shown that spectrum utilization efficiency increases as the cell size is made smaller. Although systems employing many small cells are inherently more complex, the direction of technology makes the trade-off of complexity for efficiency more attractive. Since this trend will undoubtedly continue, it is reasonable to examine the potential of small-cell systems in other respects and to explore methods for further improving the efficiency of such systems.
The use of dynamic channel reassignment has been heretofore examined and improvements evaluated. It has been found that a seven-cell channel-reusage pattern is optimum for jointly reducing co-channel interference and increasing the capacity of the system. More recently, it has also been shown that twenty to sixty times as many users may be handled by such wideband systems when compared to conventional narrowband FM. Furthermore, these wideband systems are non-synchronous and require no unique frequency assignments. Since time synchronization is difficult to achieve in a mobile system, and since dynamic channel assignments require supervisory control, there are some significant operational advantages for the wideband system.
The promising capabilities of wideband systems make it desirable to seek additional improvements in spectrum utilization by capitalizing on the nonsynchronous properties of such signals. This makes it possible to consider discrete modulation methods for speech that are not feasible with synchronous systems such as PCM.